1. Field of the Invention
The invention relates generally to single cone roller cone drill bits, and more specifically to simulating the drilling performance of single cone roller cone bits. In particular, the invention relates to methods for generating a visual representation of a single cone roller cone bit drilling earth formations, methods for designing single cone roller cone bits, and methods for optimizing the drilling performance of a single cone roller cone bit design.
2. Background Art
Roller cone bits are one type of drill bit used to drill wellbores through earth formations. Roller cone bits include a bit body adapted to be coupled to a drilling tool assembly or “drill string” which rotates the bit as it is pressed axially into the formations being drilled. FIG. 1 shows one example of a conventional drilling system drilling an earth formation. The drilling system includes a drilling rig 10 used to turn a drill string 12 which extends downward into a well bore 14.
The bit body includes one or more legs, each having thereon a bearing journal. The most commonly used types of roller cone drill bits include three such legs and bearing journals. A roller cone is rotatably mounted to each bearing journal. During drilling, the roller cones rotate about the respective journals while the bit is rotated. The roller cones include a number of cutting elements, which may be press fit inserts made from tungsten carbide and other materials, or may be milled steel teeth.
The cutting elements engage the formation in a combination of crushing, gouging, and scraping or shearing action which removes small segments of the formation being drilled. The inserts on a cone of a three-cone bit are generally classified as inner-row insert and gage-row inserts. Inner row inserts engage the bore hole bottom, but not the well bore wall. Gage-row inserts engage the well bore wall and sometimes a small outer ring portion of the bore hole bottom. The direction of motion of inserts engaging the rock on a two or three-cone bit is generally in one direction or a very small limited range of direction, i.e., 10 degrees or less.
One particular type of roller cone drill bit includes only one leg, bearing journal, and roller cone rotatably attached thereto. The drilled hole and the longitudinal axis of this type of bit are generally concentric. This type of drill bit has generally been preferred for drilling applications when the diameter of the hole being drilled is small (less than about 4 to 6 inches [10 to 15 cm]) because the bearing structure can be larger relative to the diameter of the drilled hole when the bit only has one concentric roller cone. This is in contrast to the typical three-cone rock bit, in which each journal must be smaller relative to the drilled hole diameter.
An important performance aspect of any drill bit is its ability to drill a wellbore having the full nominal diameter of the drill bit from the time the bit is first used to the time the cutting elements are worn to the point that the bit must be replaced. This a particular problem for single cone bits because of the motion (trajectory) of the cutting elements as they drill the wellbore. The inserts on a single cone bit go through large changes in their direction of motion, typically anywhere from 180 to 360 degrees. Such changes require special consideration in design. The inserts on a single cone bit undergo as much as an order of magnitude more shear than do the inserts on a conventional two or three cone bit. Such amounts of shear become apparent when looking at the bottomhole patterns of each type of bit.
A general structure for a single cone rock bit is shown in cut away view in FIG. 2. The bit includes a bit body 1 made of steel or other high strength material. The bit body 1 includes a coupling 4 at one end thereof that is adapted to join the bit body 1 to a drill string (not shown) for rotating the bit during drilling. The bit body 1 may include gage protection pads 2 at circumferentially spaced apart positions about the bit body 1. The gage protection pads 2 may include gage protection inserts 3 in some embodiments. The gage protection pads 2, if used, extend to a drill diameter 18 of the bit. Other embodiments of a bit according to the invention may not have gage pads.
The other end of the bit body 1 includes a bearing journal 1A to which a single, generally hemispherically shaped roller cone 6 is rotatably mounted. In some embodiments, the cone 6 may be locked onto the journal 1A by retaining or locking balls 1B disposed in corresponding grooves or races on the outer surface of the journal 1A and on the interior surface of the cone 6. Locking balls are only one example of a mechanism to retain the cone 6 on the journal 1A.
The cone 6 is formed from steel or other high strength material, and may in some embodiments be covered about its exterior surface with hardfacing or similar coating intended to reduce abrasive wear of the cone 6. In some embodiments, the drill bit will include a seal 8 disposed between cone 6 and journal 1A to exclude fluid and debris from entering the space between the inside of the cone 6 and the journal 1A. Such seals are well known in the art. The journal 1A and cone 6 are arranged so that the cone 6 is roughly concentric with the longitudinal axis 11 of the bit body 1. The journal 1A depends from the bit body 1 such that it defines an angle α between the rotational axis 9 of the journal 1A and the rotational axis of the bit 11. The size of this angle α will depend on factors such as the nature of the earth formations being drilled by the bit.
The cone 6 includes a plurality of cutting elements thereon at selected positions, which may be, for example, inserts 5 generally interference fit into corresponding sockets (not shown separately) in the outer surface of the cone 6. The inserts 5 may be made from tungsten carbide, other metal carbide, or other hard materials known in the art for making drill bit inserts. The inserts 5 may also be made from polycrystalline diamond, boron nitride or other super hard material known in the art, or combinations of hard and super hard materials known in the art.
One significant factor to be considered in the design of a single cone roller cone drill bit is its ability to avoid “tracking,” a situation in which cutting elements traverse the same subset of the cross-section of the drilled hole, leaving other areas of the cross-section undrilled. Tracking reduces drilling performance because the hole bottom is not evenly drilled. Avoiding tracking in single cone rock bits is particularly difficult because of the very complex motion of the individual cutting elements on the roller cone as the bit drills earth formations.
Significant expense is involved in the design and manufacture of drill bits. Therefore, having accurate models for simulating and analyzing the drilling characteristics of bits can greatly reduce the cost associated with manufacturing drill bits for testing and analysis purposes. For this reason, several models have been developed and employed for the analysis and design of 2, 3, and 4 roller cone bits. See, for example, U.S. Pat. Nos. 6,213,225, 6,095,262, 6,412,577, and 6,401,839. In addition, U.S. Pat. No. 6,516,293 discloses a simulation method for multiple cone bits, which is assigned to the assignee of the instant application, and is incorporated by reference in its entirety.
The simulation model disclosed in the '293 patent is particularly useful in that it provides a means for analyzing the forces acting on the individual cutting elements on the bit, thereby leading to the design of, for example, faster drilling bits and designs having optimal spacing and placing of cutting elements on such bits. By analyzing forces on the individual cutting elements of a bit prior to making the bit, it is possible to avoid expensive trial and error designing of bit configurations that are effective and long lasting.
However, modeling single roller cone bits is significantly more complex than multiple roller cone bits because of the complex motion (explained above) of the individual cutting elements on the single roller cone as the bit drills the earth formation.
What is needed are methods to simulate and optimize performance of single cone roller cone bits drilling earth formations. Simulation of single cone roller cone bits would enable analyzing the drilling characteristics of proposed bit designs and permit studying the effect of bit design parameter changes on the drilling characteristics of a bit. Such analysis and study would enable the optimization of single cone roller cone drill bit designs to produce bits which exhibit desirable drilling characteristics and longevity. Similarly, the ability to simulate single cone roller cone bit performance would enable studying the effects of altering the drilling parameters on the drilling performance of a given bit design. Such analysis would enable the optimization of drilling parameters for purposes of maximizing the drilling performance of a given bit.